I've decided I want to learn a web programming language. I've been reading up a bit, and PHP looks like the primary candidate. If I concentrate on version 5, is there enough commonality and backward compatibility to permit me to customise a PHP4 application we use at work, or would I be better to learn PHP4 first?

You should find that 99% of what you do will be backwards compatible, now that 5 has matured a bit I would start off with that. Most ISPs still use 4.3 / 4.4 series but that will change over the next year or 2.